Friction mechanisms containing silicon nitride

ABSTRACT

A friction mechanism comprising an assembly of two relatively rotatable friction members, wherein the engaging surfaces of both friction members are provided with a sintered friction material, at least one of the friction members containing silicon nitride as a constituent. The friction material may also contain graphite and other friction and wear modifying agents such as molybdenum disulphide tin and lead, of which the following is a specification.

United States Patent Bray [451 Nov. 21, 1972 FRICTION MECHANISMSCONTAINING SILICON NITRIDE Inventor: Horace A. Bray, Coventry, EnglandThe Dunlop Company Limited, London, England Feb. 27, 1970 Assignee:

Filed:

Appl. No.:

[30] Foreign Application Priority Data U.S. Cl. ..188/251 M, 29/1825,75/205, 192/107 M Int. Cl ..Fl6d 69/02 Field of Search ..l88/25l M;192/107 M; 75/.5 AC, .5 BC, 205; 29/1825 Dasse ..I88/251 M 3,376,9604/1968 Bender ..188/251 M X FOREIGN PATENTS OR APPLICATIONS 1,052,63612/1966 Great Britain ..l88/251 M Primary Examiner-George E. A. HalvosaAttorney-Jeffers and Young [5 7] ABSTRACT A friction mechanismcomprising an assembly of two relatively rotatable friction members,wherein the en- V gaging surfaces of both friction members are providedwith a sintered friction material, at least one of the friction memberscontaining silicon nitride as a constituent. The friction material mayalso contain graphite and other friction and wear modifying agents suchas molybdenum disulphide tin and lead, of which the following isa-specification.

5 Claims, 2 Drawing Figures 7 l5 l5 2 l7 I5 FRICTION MECHANISMSCONTAINING SILICON NITRIDE This invention relates to frictionmechanisms, and particularly, but not exclusively to brakes for use inaircraft.

An object of the invention is to provide an improved friction mechanismfor use in, for example, aircraft brakes, tractor and earthmover brakes,steering clutches etc., and similar high energy dissipating mechanisms.

In UK. Pat. specification No. 1,052,636 we have described and claimedimproved friction elements for use in brakes, clutches or the like,containing silicon nitride dispersed in a suitable matrix, and suchfriction elements have been used successfully in friction mechanisms inwhich the friction elements are mounted on one of two relativelyrotatable friction members for engagement with the other friction memberof which the surface at least is formed from a metal such as steel.

According to the present 'invention, a friction mechanism comprises anassembly of two relatively rotatable friction members the engagingsurfaces of both friction members being provided with a sintered'friction material at least one of the friction members containingsilicon nitride as a constituent.

The concentration of silicon nitride in the sintered friction materialof a friction mechanism according to the invention may be between 0.5and 70 percent by weight of the friction material and preferably the concentration lies in the range 0.5 to 20 percent by weight of the frictionmaterial.

One embodiment of the invention will now be described by way of examplewith reference to the accompanying drawings of which:

FIG. 1 shows a cross-sectional view of one kind of aircraft disc brakein which friction mechanisms in accordance with the present inventionare employed, the cross-sectional view being taken on a plane at rightangles to theplanes of the brake discs and FIG. 2 shows part of a rotorand stator for use in a brake of the kind shown in FIG. 1.

A friction mechanism in the form of an aircraft brake 1 (see FIG. 1)comprises an assembly of relatively rotatable friction membersconstituted by a series of annular rotor 2 and stator 3 brake discsmounted coaxially with and in surrounding relation to the hub of anaircraft wheel.

The rotor and stator discs are mounted in interleaved relationship on atorque tube 4 projecting axially from a torque member 5 secured to thenon-rotatable wheel hub 6.

Each rotor disc 2 is keyed to the aircraft wheel by means of drive dogs7 at the radially outer circumference of the disc engaging incorresponding keyways (not shown) formed in the wheel so that the discis axially movable relative thereto.

Apart from the axially outermost stator disc 8, each stator disc 3 iskeyed to the torque tube 4 by means of drive dogs 9 (see FIG. 2) at theradially inner circumference of the disc engaging splines 10 formed onthe tube, so that the disc is axially movable relative to the tube. Theaxially outermost stator disc 8 is mounted in an axially fixed positionon an annular reaction member 11 in the form of a flange secured to thetorque tube at the axially outermost a end thereof.

Means for urging the rotor and stator discs into engagement with oneanother is provided in the form of a series of piston and cylinderassemblies 12 mounted on the torque member so that the pistons thereofare engageable with the axially innermost stator disc 13.

When fluid is supplied to the cylinders, the axially movable rotor andstator discs are moved by the pistons axially outwardly until the statordisc on the reaction member prevents further movement, and the rotordiscs are clamped between the stator discs in frictional engagementtherewith.

Each rotor disc 2 comprises an assembly of steel segments 14 (see FIG.2) which interlock in the manner of jig-saw pieces. A rotor disc of thiskind is more fully described in ou'r'U.K. Pat. specification No.931,031. Two friction elements 15 in the form of moulded pads ofsintered friction material are located one on each side of each segmentof the rotor disc and bonded directly thereto.

The axially movable stator discs 3 are each in the form of a rigid steelannulus 16 (see FIG. 2) having a plurality of friction elements 17secured thereto at closely circumferentially-spaced positions on bothsides thereof. Each stator friction element comprises a moulded pad ofsintered friction material bonded to a steel backing plate which itselfis rivetted 18 to the rigid steel annulus.

The stator disc 8 which is secured to the reaction member 11 comprises aplurality of friction elements 19 rivetted to an annular steel backingplate 20 which is itself secured to the reaction member 11. The frictionelements are constructed and arranged in a manner similar to the statorfriction elements described in the last preceding paragraph.

The axially innermost stator disc 13 is constructed and arranged in amanner otherwise similar to the other axially movable stator discs, butis provided with friction elements 21 on its axially outermost side faceonly, its other side face 22 being formed to co-operate with thebrake-applying pistons 12.

The moulded pads of sintered friction material for both the rotor andstator discs are formed by mixing silicon nitride in powder form withiron powder, powdered graphite and other friction and wear modifyingagents, and sintering the mixture in a non-oxidizing atmosphere, as ismore fully described in our UK. Pat. specification No. 1,052,636. Thefinished pads, ready for incorporation in the brake have a compositionwithin the following ranges, in which the iron constitutes both thematrix and a friction-modifying constituent of the friction material:

Percentage by weight constituent of finished pad Silicon nitride 0.5 to5% Graphite S to 10% Other friction and wear 0 to 5% modifying agentsIron Remainder The other friction and wear-modifying agents referred toabove include, for example, lead, molybdenum disulphide and tin, andthese materials are used to impart specific properties to the frictionmaterial in the manner, well-known to those skilled in the art, in whichthey are used in conventional friction compositions.

A typical specific formulation for a friction material for incorporationin the brake described above is as follows:

Percentage by weight Constituent of finished pad Iron 85 Silicon Nitride2.5 Lead 1 Graphite 8 Molybdenum Disulphide l Tin 2.5

It has been found that an aircraft brake as described above exhibits anoutstanding improvement in performance compared with otherwise similarbrakes in which the sintered friction material in the rotor or statordiscs is replaced by sintered friction materials not containing siliconnitride or by a steel or iron surface, or in which the sintered frictionmaterial on both rotor and stator discs is replaced by another sinteredfriction material not containing silicon nitride. This improvement isthought to arise from the synergistic cooperation of thesilicon-nitride-containing friction surfaces.

Friction pads with compositions as described in the above specificformulation have been tested in an aircraft type disc brake on aninertia wheel dynamometer with the following results.

Energy Absorption Coefficients of friction level ft.lbs./lb./sec.

Example l Example 2 3000 .26 .21 5000 .25 .20 7000 .22 .19 8000 .19 .17

Example 1 shows the variation of coefficient of friction with energyloading for a brake in which both rotors and stators are provided withpads whose compositions are as described in the above specificformulation.

Example 2 shows the variation of coefficient of friction with energyloading for a brake in which either the rotors or stators are faced withsteel.

These results clearly show the improvement in coefficient of frictionderived from synergistic co-operation when both rotors and stators areequipped with pads containing silicon nitride.

Having now described my invention what I claim is:

1. A friction mechanism comprising an assembly of two relativelyrotatable friction members having confronting engageable surfaces andwherein the engaging surfaces of both friction members are each providedwith a sintered friction material containing silicon nitride as aconstituent to provide a cumulative increase in coefficient of frictionof not less than 10 per cent as compared with friction members having acombination of silicon nitride and steel as the confronting engageablesurfaces of the friction members.

2. A friction mechanism according to claim 1 wherein the composition byweight of the friction material containing silicon nitride comprises 0.5percent silicon nitride, 5 to 10 percent graphite and O to 5 percent ofother friction andwear modifying agents, the remainder of the materialbeing iron.

3. A friction mechanism according to claim 1 wherein the composition byweight of the friction material containing silicon nitride comprises 2.5percent silicon nitride, 8 percent graphite, percent lead,

1. A friction mechanism comprising an assembly of two relativelyrotatable friction members having confronting engageable surfaces andwherein the engaging surfaces of both friction members are each providedwith a sintered friction material containing silicon nitride as aconstituent to provide a cumulative increase in coefficient of frictionof not less than 10 per cent as compared with friction members having acombination of silicon nitride and steel as the confronting engageablesurfaces of the friction members.
 2. A friction mechanism according toclaim 1 wherein the composition by weight of the friction materialcontaining silicon nitride comprises 0.5 percent silicon nitride, 5 to10 percent graphite and 0 to 5 percent of other friction and wearmodifying agents, the remainder of the material being iron.
 3. Afriction mechanism according to claim 1 wherein the composition byweight of the friction material containing silicon nitride comprises 2.5percent silicon nitride, 8 percent graphite, 1 percent lead, 1 percentmolybdenum disulphide, 2.5 percent tin and 85 percent iron.
 4. Afriction mechanism according to claim 1 wherein the friction memberscontain silicon nitride in the amount of 0.5 to about 70 percent byweight of the friction material.